Soybean rust discovered in the United States last week in two small plots in Louisiana may have spread to three more fields in the state and one in neighbouring Mississippi, according to federal and state agriculture investigators. The disease is caused by two fungal species known as Phakopsora pachyrhizi and Phakopsora meibomiae. The type found in Louisiana was the Asian species, Phakopsora pachyrhizi, which is more aggressive and damaging than the other.
The disease is hard to identify in early stages when symptoms appear as yellowed leaves. As the disease progresses, brown or reddish pustules appear on the bottom surface of the leaf. Infected plants quickly shed their leaves, reducing soybean pod set and fill, which means deteriorating crop yields and seed quality. The fungus has devastated crops in many parts of the world, with soybean yield losses up to 80 percent in some areas of Africa and South America.
Although frost kills the fungus, in mild climates its spores can survive by attaching to host plants such as legumes.
Soybean rust can be carried aboard ships or airplanes, but it is usually spread by windborne spores. The USDA theorised that this year's active hurricane season blew the disease north from South America into the US Gulf Coast.
In 1998 in Africa, spores were blown from Uganda to Zimbabwe, according to the USDA. Between 2001 and 2003, the disease spread to South America, quickly travelling the length of the continent and infecting as much as 90 percent of Brazil's soybean acres on the way.
The fungus can travel about 20 to 30 miles a day, depending on wind patterns and weather conditions. Now that it has been discovered in Louisiana, it may spread to large soy-producing states in the Midwest by the 2005 growing season.
Quarantine of an infected area is useless because the disease is spread by the wind.
The fungus is not a health risk for consumers.
The USDA said soybean rust is unlikely to hurt US exports of soybeans and soy products, which totalled some $10.9 billion in fiscal 2004.
China, Brazil and many other countries with soy rust have continued to export soybeans.
The US farmers harvested nearly 74 million acres of soybeans in 2004, with the biggest production coming from Iowa and other Midwestern states.
Farmers must monitor soybean fields and adjacent areas throughout the growing season. If soybean rust is detected early, fungicides can be applied to plants to reduce yield loss. The cost of fungicides is estimated at about $25 per acre, which would boost the cost of producing US soybeans by 15 to 20 percent.
The long-term solution is to develop new varieties of soybeans that are resistant to rust. Such varieties are in the works but will not be available to farmers for at least five years.
Sources: USDA, Iowa State University, American Soybean Assn.